1. Field of the Invention
This invention relates generally to the measurement of the amount and/or size of gas bubbles entrained in building materials such as concrete and asphalt, while said materials are still in a viscous or pliable state.
2. Prior Art
It is desirable to be able to determine the amount and size of air voids (bubbles) in concrete. With increasing total air content, there is an increase in freeze-thaw durability, workability, pumpability, adhesion, and sulfate resistance, but a decrease in strength. Void size also affects these properties. It is thus desirable to maintain a controlled amount and size of air voids in concrete.
The following prior art techniques are employed to measure the total volume of entrained air in concrete. They all have drawbacks and limitations.
ASTM C138 test determines air content as the difference in weight of a known volume of sample from its theoretical airfree weight. Even a small error in measured parameters (batch proportions, specific gravity, sample volume or sample weight) yields a significant error in results. The necessary parameters are difficult to obtain accurately in the field.
ASTM C231 test entails measuring the volume reduction of the concrete slurry upon application of pressure to a sample. This is accomplished by either measuring the volume change (compression) of the sample, or by measuring the final air pressure after releasing a known quantity of compressed air into the chamber. The test chamber must be rugged and pressure tight, and therefore expensive.
ASTM C173 test measures the volume reduction of the wet concrete upon dispelling the air through the addition of water and alcohol followed by agitation. This method is laborious and occasionally must be repeated due to insufficient air detrainment. Efforts to enhance the air detrainment with vacuum resulted in a more tedious procedure.
The Air-Master Meter is a smaller, plastic version of the ASTM C173 apparatus. It suffers from the same problems as does ASTM C173, but to a lesser extent.
Chase Air Indicator is a much smaller (4 cc volume) version of the ASTM C173 apparatus, for measuring only the mortar portion of concrete. Its results are prone to error due to small sample size, improper separation of the mortar portion, and improper calibration.
The aforementioned methods are limited to concrete specimens. The following technique has been developed to determine total air content of concrete in-place (in-situ).
The thermal method entails pushing a bottomless chamber into the in-place concrete. A heater within the chamber heats the concrete, causing the sample to expand in proportion to its air content, which, in turn, produces a proportionate air pressure rise. The measured air pressure in the chamber is then related to the air content. The accuracy of this test has not been independently verified.
The following techniques have been developed to determine void size, but are not being used by the industry.
The Void Spacing Indicator Method entails dispersing the concrete in water and observing the size of the detrained air bubbles that rise to the surface. The accuracy of the method is very poor.
The vibration method entails vibrating the sample to detrain the air and graphing volume reduction versus time. This method is based on the principle that larger voids rise to the surface faster than small ones. A complex mathematical formula yields void size from the data. The accuracy of this method has not been determined.
Thus, the present tests for measuring entrained air and void size in concrete all have serious limitations.